Magnetic tape used for data recording typically is one of two types: helical tracks (for example, Digital Audio Tape) in which tracks are written oblique to the direction of movement of the tape, or linear tracks, in which tracks are written parallel to the direction of movement of the tape. For linear tracks, a common drive configuration has a magnetic head with a single read/write gap positioned on a data track such that a head gap centerline is within a prescribed dimensional tolerance of the recorded track centerline. Positioning may be open loop, or track positioning information may be included on a tape for closed loop positioning.
There are two head positioning parameters of particular interest to the present application. One parameter is vertical position accuracy for the magnetic head. Typically, vertical position is controlled by a lead screw driven by a stepper motor, with many motor steps required to change the head position from one track to another. The second parameter of interest is the angle of the magnetic head relative to a line that is transverse to the direction of tape motion. If the line transverse to the direction of tape motion is defined as vertical, azimuth is the angle of the head, in the plane of the tape, relative to vertical, and zenith is the angle of the head, in a plane transverse to the plane of the tape, relative to vertical. Ideally, azimuth and zenith are zero.
One track positioning approach is illustrated by U.S. Pat. No. 4,747,004 (Kukreja et al.). In Kukreja et al., head azimuth and zenith are determined by the angle of the lead screw (pin 38 functions primarily to prevent rotation of the head carriage around the lead screw (yaw)). However, for lower drive height, it is useful to move the stepper motor off-axis from the lead screw, and for lowest cost, it is useful to allow the lead screw to adapt to misalignment instead of requiring a precision mount for the lead screw. See, for example, U.S. Pat. No. 5,537,275 (Peace et al.), which is incorporated herein by reference for all that it discloses and teaches. In Peace et al., the design permits the lead screw to tilt relative to a lower plate and to tilt relative to a upper plate, which in turn relaxes alignment requirements for the lead screw. Azimuth and zenith are then controlled by a separate low cost guide pin. The arrangement shown in Peace et al. reduces cost and lowers drive height, but introduces a hysteresis problem, as illustrated in FIGS. 1B-1C.
In FIG. 1A, a magnetic head is attached to a rigid arm 102. Arm 102 is driven by a lead screw 104 and a follower nut 106 threaded onto the lead screw. A guide pin (or functional equivalent) 108 helps keep the arm 102 aligned in azimuth and zenith. When the lead screw 104 rotates, the arm 102 moves vertically. The arrangement illustrated in FIG. 1A has an inherent limitation, as illustrated in FIGS. 1B and 1C. In FIG. 1B, the lead screw is rotating such that the arm 102 is moving vertically. However, due to mechanical tolerances and friction between the arm 102 and the guide pin 108, the arm pivots slightly around the guide pin (exaggerated in FIG. 1B for purposes of illustration) so that when the follower nut 106 is pushing the arm upward, the magnetic head 100 tilts down relative to the follower nut. When rotation of the lead screw is reversed, the arm 102 pivots in the opposite direction, as illustrated in FIG. 1C. As a result, when rotation of the lead screw 104 first reverses to cause movement of the head 100 to reverse from upward movement to downward movement, the head 100 actually continues to move upward as the arm 102 reverses its pivot angle around the guide pin 108. Therefore, as the lead screw reverses, there is hysteresis in the vertical movement of the head 100. There is a need for a head positioning device with low cost and reduced hysteresis.